1. Field of the Invention
The present invention is directed to a transmitter optical sub-assembly, in particular, a transmitter optical sub-assembly including a laser source having an associated laser driver.
2. Brief Description of Related Developments
Transmission speed is an ever-increasing parameter in data communication systems. Achieving and ensuring the required performance levels of such systems with conventional methods of packaging optoelectronic components are increasingly difficult at transmission speeds of 10 Gbit/s and higher.
This is particularly true for optical communication transceivers for use in systems where large volumes of data are aggregated to form serial data streams with very high rates. These data streams are subsequently used to drive light sources such as laser sources. As the transmission speed/rate increases, apparatus such as optical transmitters intended to have small dimensions and required to be produced at low cost is exposed to significant issues in terms of thermal management and signal integrity.
At transmission rates of 10 Gbit/s and higher, the existing solutions for producing a transmitter optical sub assembly (TOSA) are particularly exposed to critical operating conditions. This applies to both basic types of TOSA. arrangements currently adopted.
A first type of known TOSA arrangement is shown in FIGS. 1 and 2. These figures refer to TOSA packages of the types currently referred to as TO-CAN and planar, respectively. Either type of arrangement includes a laser diode driver LDD and a laser diode LD assembled in the same package PKG that is traversed by electrical lines L. These types of arrangement are advantageous in that the connections between the laser driver LDD and the laser diode LD may be minimized.
The main drawback of this arrangement lies in that heat dissipated by the driver is transferred to the laser diode by conduction, convection and IR emission within the common package. Laser diode performance degrades with increasing temperature, and careful thermal management is needed in order to guarantee the desired performance. Thermal management is usually effected by cooling the laser by means of Peltier devices, which however add to the overall power consumption while also leading to additional costs and dimensions of the module.
Alternatively, laser diodes may be employed that are adapted to operate at higher temperatures, but this has a negative impact in terms of yield and overall costs of the transmitter. Extensive use of thermally conductive materials, which may also help in the circumstances outlined in the foregoing, inevitably entails higher costs.
Another type of known TOSA arrangement is exemplified in FIG. 3. There, the laser source LD is located on an optical bench OB and packaged alone within the package PKG. The laser driver is placed on a printed circuit board PCB, outside the package.
This latter solution is advantageous in terms of thermal management. Designing the electrical connections between the laser diode and the driver is however more complicated and less effective. Impedance mismatch between the laser driver and the laser diode creates electrical reflections that lead to impairment of electrical performance, particularly at high bit rates.
This problem could be avoided by adding in series with the laser diode a resistor to match the driver impedance. However, in such an arrangement, a part of the signal power is lost due to the voltage drop across the matching resistor. Also, the laser driver output swing is limited and related to the supply voltage; this solution can thus be resorted to only by using a higher supply voltage (5 V or more). This choice leads to higher power dissipation and adds to the system complexity, since all the other circuits in a transceiver are usually fed from a 3.3 V power supply.